1. Field of the Invention
The present inventions relate generally to detection of radiation, such as infra-red radiation.
2. Description of Background Art
A photodiode is an electrical component that behaves as a photodetector. Photodiodes are typically implemented as p-n junction diodes that are responsive to optical input, for example, by providing a window, fiber, or other means for photons to impinge on a light-sensitive part of the device.
Many types of photodiodes operate in reverse bias mode. Diodes typically have high impedance when reverse biased, and light of a proper frequency can generate mobile carriers which are detected in the high impedance diode circuit, thereby allowing sensitive measurement of the light.
FIG. 1A depicts an energy band diagram of a basic p-n junction photodiode. In this diagram, electron energy is plotted vertically, while spatial distance is plotted horizontally. The diagram includes conduction band 102, valence band 104, and Fermi energy level 106. The distance between valence band 104 and conduction band 102 is called the energy gap, or bandgap, 108. In this example, p-type semiconductor 110 is on the right, while n-type semiconductor 112 is on the left. Junction region or depletion region 114 is shown near where the p-type region 110 and the n-type region 112 meet. An electric field E 116 exists in this region. The field 116 exerts a force on charge carriers, moving holes to the right and electrons to the left.
FIG. 1B shows a p-n junction photodiode under reverse bias, a typical operating condition. As in FIG. 1A, the electric field 116 is a result of the formation of a junction or depletion region between 114 the two sides. If a photon 118 with energy greater than bandgap energy 108 strikes regions 110, 112, or 114, electron and hole pairs may be generated by absorption of the photon, which excites an electron from the valence band 104 to the conduction band 102. In typical p-n junction photodiodes, region 114 with electric field 116 is considered the photon absorption region of the photodiode, together with those parts of regions 112 and 110 within a minority carrier diffusion length of region 114. The two outer regions 112, 110, are considered contact regions for collecting photogenerated carriers. Depletion region 114 combined with minority carrier diffusion length regions of 112 and 110 constitute the photon absorption region.
Because the same material is used throughout the device in this example, the bandgap has a constant value across the junction. Such a junction is known as a homojunction, because the junction between semiconductors differs only in doping levels, and not in alloy or atomic composition.
FIG. 1C shows a similar p-n junction diode, this one comprising a heterojunction wherein the bandgap between p-type and n-type regions do not match. In this example, the valence bands 104 of the two halves match, but the conduction bands 102 show a discontinuity 120 where the different bandgap materials meet. The bandgap 108A of the p-type region 110 is smaller than the bandgap 108B of the n-type region 112. The discontinuity 120, in this example, provides a potential barrier against minority carrier flow from the p-type region to the n-type region, even under reverse bias. In this Figure the band-bending is explicitly shown, unlike in the band diagrams of FIGS. 1A and 1B, and hence the valence and conduction band levels are slightly curved at transition points.
Photodiodes, like other electrical devices, experience noise on the current signal. Noise in a photodetector can arise from a combination of sources, including Auger, Shockley-Read-Hall, radiative recombination, and background photon flux. When a photodetector has no light input, the output of the photodetector is called dark current, which consists mainly of diffusion currents from either side of the junction, depletion current from S-R centers in the depletion region of the diode, and some contribution from tunneling currents. For temperatures such that nmaj>ni, (where nmaj is the majority dopant concentration on either side of the junction, and ni is the intrinsic carrier concentration) the overall dark current of a well-made diode is dominated by the S-R centers in the depletion region of the diode.
One driving requirement in photodetectors is to reduce dark current. The ideal photodetector cell should produce an electrical signal which increases with increased illumination, but which drops to zero when there is no illumination. This is not normally possible in practice. Dark current not only provides a background “noise” signal which makes images less understandable, but also increases the power consumption of the device.
One of the causes of dark current is the aforementioned Shockley-Read-Hall (S-R) current from generation/recombination centers in the semiconductor's crystal lattice. Any discontinuity or foreign atom in the crystal lattice can provide a location where carrier pairs (electron+hole) can be generated. These S-R centers generate diffusion currents in the electric field-free regions of the diode as well as depletion generation-recombination (g-r) current in the diode depletion region.
Another source is Auger generation. A “hot” carrier (electron or hole) is one which has more energy than the minimum for its band. In p-type material, if a “hot” hole has more than a bandgap's worth of excess energy, it can share its excess energy to promote a valence electron to the conduction band. This electron is a minority carrier in p-type material, and will be a component of dark current. In n-type material, if a hot electron has more than a bandgap's worth of excess energy, it can share its excess energy to similarly generate a mobile hole, which as a minority carrier can contribute to dark current. (This is a very brief summary of the “Auger7” and “Auger1” processes, which are extensively described in the technical literature.) An important difference between these processes is that the time constant for the Auger7 process (in p-type material) in materials systems with a direct energy gap at k=0 is roughly 10-50 times longer than the time constant for the Auger1 process (in n-type material). Thus the Auger component of dark current is much less important in p-type material, if other factors are comparable.
Reduction in dark current is highly desirable. One way to exploit a reduction in dark current is to reduce cooling requirements: infrared imagers are often cooled below room temperature, e.g. with a thermoelectric cooler (“cold finger”), or even with cryogenic refrigeration systems. However, such cooling requirements—especially if cooling much below 0 C is required—add greatly to system cost, bulk, power consumption, and complexity.
Dark current can be particularly problematic for infrared detectors, since the bandgaps of the absorbing material in such detectors are relatively small, in order to permit excitation of carriers into the conduction band by absorbing infrared wavelength photons (which are of relatively low energy).
At very low temperatures, even the shallowest acceptor dopants (which ionize completely at room temperature) may not be completely ionized. For example, if arsenic is used as the acceptor dopant in HgCdTe, then at 25 K only about one tenth of the arsenic atoms will be ionized. That is, instead of an As-ion on a tellurium lattice site (and a corresponding mobile hole), the semiconductor lattice merely contains an non-ionized arsenic atom on the tellurium site. This is true even if the dopant atoms have been fully activated, i.e. are located on the correct lattice sites and also are not interstitials.
More information on the HgCdTe materials system, and on devices made from this and competing infrared detection technologies, can be found at http://en.wikipedia.org/wiki/HgCdTe, and other articles which this page links to (directly or indirectly), all of which (in their versions as of the effective filing date of the present application) are hereby incorporated by reference.
Separate Absorption and Detection Diode
The present innovations include a detector system, preferably for infra-red wavelengths of electromagnetic radiation, embodied in a diode architecture. In one example embodiment, the present innovations are described as a separate absorption and detection diode, such that photons absorbed in one region of a diode structure (e.g., a P region) are detected in another region of the diode structure (e.g., an N region). In an example embodiment, the diode is a heterostructure device including a P region which is heavily doped with respect to an adjacent N region, such that the depletion region is substantially confined to the N region of the device. An N+ region is preferably located adjacent to the N region. In preferred embodiments, the N and N+ regions have wide bandgaps relative to the P region. Further, the innovative diode structure preferably includes a graded composition in the N region near the P region such that the resulting field associated with the graded bandgap can be overcome by a modest reverse bias voltage applied to the diode. In preferred embodiments, the detection region bandgap is chosen to be large enough that the dark current is limited by thermal generation from the absorbing volume, and also large enough to eliminate tunnel currents in the wide gap region of the diode.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:                wide bandgap (of, e.g., the N region) enables a significant reduction in dark current;        graded composition permits conduction band carriers to move from absorption region to a detection region;        wide bandgap nature enables elimination of tunnel currents;        p-type nature of absorption region (in preferred embodiments) enables significant reduction in dark current;        wide bandgap nature of diode depletion region enables significant reduction in dark current;        gradient of composition bandgap in transition region is large enough to reduce dark current component from the transition region.        